This invention relates to a cutting insert, intended for drills, of the type that comprises a cutting edge from which a concavely curved, chip removing surface extends up to a ridge, where the curved surface transforms into a top side of the cutting insert. The invention also pertains to a method of chip-forming drilling.
When drilling in workpieces of, for instance, metal, there is a general desire that the chips cut free should be as short as possible in order to be evacuated from the hole being produced in the easiest conceivable way. If the chips become long, like a lock of curly hair, a risk of chip jamming arises. These difficulties become particularly marked in connection with long hole drilling in materials that form long chips, i.e., sticky or soft metals. Examples of metals that form long chips are steel with low carbon content and so-called duplex materials (“duplex” is used as a comprehensive term by those skilled in the art for different types of stainless materials that are particularly difficult to machine). By “long hole drilling” is usually understood drilling of holes the length of which is at least 5 or 10 times larger than the diameter (in extreme cases, the hole length may amount to more than 100 times the diameter). Long hole drilling may be carried out with many different types of drills, which however have in common that they, in addition to a drill head having replaceable cutting inserts, include a tubular shank, through which a cooling and/or lubricating medium may be led up to the drill head and then be transported away along with the separated chips. The supply of cooling/lubricating medium, as well as the evacuation of the same along with the chips, may be taken care of according to many different techniques that have been developed for decades. In all types of long hole drills, however, the channels for the chip evacuation are long and narrow. This means that there is a great risk of chip jamming if the chips, which are cut free, become too long.
Previously known cutting inserts for long hole drills of the above-related kind have without exception, been made with smooth chip removing surfaces, i.e., the concavely curved surface that extends from the cutting edge up to the back where the chip removing surface transforms into the top side of the cutting insert has always, in all essentials, been even and smooth. This means that the separated chip, irrespective of thickness (i.e., independent of the feeding in question) has been of uniform thickness with a smooth bottom side. That characteristic has turned out to aggravate (resist) the breaking of the chip into shorter pieces. Furthermore, the cooling liquid has problems traveling under the chip to reach the cutting edge.